A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In IC fabrication, the continuing improvements in microprocessor speed, memory packing density and low power consumption for micro-electronic components require a continuing decrease of size of the patterns that are transferred from the patterning device (e.g. mask) to the substrate by lithographic apparatus. However, as the size of an integrated circuit is reduced and its density increases, the CD (critical dimension) of its corresponding patterning device (e.g. mask) pattern approaches the resolution limit of the lithographic apparatus. The resolution of lithographic apparatus is defined as the minimum feature that the exposure tool can repeatedly expose on the substrate (e.g. wafer). Various techniques, known as resolution enhancement techniques, have been applied in order to extend the resolution limit of the lithographic apparatus.
One technique to improve resolution is off-axis illumination. With this technique, the patterning device (e.g. mask) is illuminated at selected non-perpendicular angles which may improve resolution, and particularly improves the process latitude by increasing the depth of focus and/or contrast. The angular distribution of the radiation beam at the patterning device (e.g. mask) plane, which is an object plane, corresponds to a spatial distribution of the radiation beam in a pupil plane of optical arrangements of the lithographic apparatus. Typically, the shape of the spatial distribution in a pupil plane is referred to as illumination mode. One known illumination mode is annular, in which the conventional zero order spot on the optical axis is changed to a ring-shaped intensity distribution. Another mode is multipole illumination in which several spots or beams are produced which are not on the optical axis. Examples of multipole illumination modes are dipole, including two poles, and quadrupole, including four poles. For illumination modes such as dipole and quadrupole, the size of the poles in the pupil plane can be very small compared to the total surface of the pupil plane. Consequently, all of the radiation used for exposing the substrate traverses the various optical elements at or near the pupil planes at the locations of these poles only. The optical element maybe, for example, a lens element. A fraction of the radiation traversing the lens element is absorbed by the lens element. This leads to a non-uniform heating of the lens element by the radiation beam, resulting in a local change in refractive index and a deformation of the lens element. The local change in refractive index and deformation of this lens element results in optical aberrations and a distorted image as projected by the projection system onto the resist layer. Local changes in refractive index and deformation of the lenses may alternatively, or additionally, be a symptom of aging.
It is desirable to provide, for example, a lithographic apparatus and method which obviate or mitigate one or more of the problems of the prior art, whether identified herein or elsewhere.